Neuropeptide Y (NPY) modulates cardiovascular, feeding and reproductive functions. Peripheral neurohumoral inputs from these systems are integrated and transformed into efferent signals in the brainstem. However, detailed mapping of NPY-expressing cells in the brainstem has not been established in primates. We have previously shown that central infusion of NPY stimulates a massive release of hypothalamic gonadotropin-releasing hormone (GnRH), suggesting that increased NPY gene expression, synthesis and release may be responsible in part for the initiation and/or maintenance of the estradiol-17 (E2)-induced preovulatory GnRH surge in primate species. Bi-directional neurohumoral communication between the brainstem and hypothalamus has been well established, and E2 receptor binding has been demonstrated in the monkey brainstem. Thus, we hypothesize that exogenous E2 treatment, which is known to induce a preovulatory-like surge of luteinizing hormone (LH), will increase the expression of NPY mRNA in the brainstem. To test this hypothesis, we utilized the in situ hybridization method to identify and quantitate NPY mRNA in ovariectomized rhesus macaques without or with E2 treatment. Individual brains were fixed with paraformaldehyde before (n=5) or at 20 h (n=3) and 30 h (n=5) after the E2 treatment time intervals that span the initiation and peak plasma LH surge. In the brainstem, hybridized images of NPY mRNA were found in the nucleus of the solitary tract, the dorsal motor nucleus of the vagus nerve, the nucleus of the spinal tract of trigeminal nerve, the lateral reticular nucleus, the reticular formation, the locus coeruleus, the nucleus of raphe and the lateral tegmentum surrounding the periaqueductal grey. To compare the expression of NPY mRNA between E2 treated and untreated animals, hybridized signals were transferred onto Cronex X-ray films and digitized with NIH image software. Signals/cell were quantified by a computer-assisted image analysis program. Examination of the locus coeruleus shows that the optical densities of NPY mRNA expression after 20 h (113 q 21) and 30 h (128 q 17) were significantly (p< 0.01) higher than those in E2-untreated animals (23 q 11). Comparison of NPY mRNA expression in other brain areas is in progress. These data suggest that estrogen may enhance gene expression in brainstem NPY neurons. The up-regulated mRNA expression in the locus coeruleus may be an important component in initiating and/or sustaining the preovulatory GnRH release in the hypothalamus.